Agricultural tractors which, as traction vehicles, are also configured for pulling trailers, frequently have infinitely variable transmissions, in particular hydrostatic-mechanical transmissions. Such infinitely variable transmissions make it possible to decelerate the traction vehicle solely by activating an operator control lever (joystick) in order to change the transmission ratio without the accelerator pedal or the service brake having to be activated for this purpose. Additionally or alternatively to this, in the case of traction vehicle-trailer combinations which differ from vehicles which are used agriculturally the traction vehicle-trailer combination can be decelerated without activating the service brake and solely by activating the accelerator pedal of a drive machine of the traction vehicle in the direction of relatively low velocities.
When the service brake is not activated such deceleration has the consequence that the trailer is not braked and therefore pushes in an unbraked fashion onto the traction vehicle. This can give rise to high coupling forces between the traction vehicle and the trailer and therefore to a certain extent to instability of the traction vehicle-trailer combination. In particular, driving situations which are critical or unstable in terms of vehicle movement dynamics, in particular when traveling uphill or on a negative gradient or even on roadways with a low coefficient of friction can be brought about if the pushing trailer causes the traction vehicle-trailer combination to jackknife.
As a result of increasing use of the infinitely variable transmissions described above in agricultural vehicles and as a result of the frequently encountered operator control behavior of drivers who predominantly use the infinitely variable transmission to decelerate the vehicle instead of the service brake, undesired pushing of the trailer onto the traction vehicle and situations which are critical in terms of vehicle movement dynamics occur more frequently. This situation is exacerbated by the higher maximum speeds of agricultural traction vehicles which are found nowadays. Furthermore, the engine and the transmission are loaded relatively heavily by this type of deceleration.
Taking this problem as a starting point, there are already approaches in which a pushing operation is detected on the basis of driving operation parameters and the trailer vehicle is automatically braked when a defined pushing effect is exceeded. This gives rise to a reduction of the coupling forces and therefore to stabilization of the traction vehicle-trailer combination. The parameters which are to detect a pushing operation are the engine rotational speed of the drive machine of the traction vehicle or the pressure in infinitely variable hydrostatic transmissions.
A genus-forming method is discussed in EP 2 269 880 A1. In the method, a measure for the pushing effect of the trailer and/or a parameter for detecting the pushing of the trailer onto the traction vehicle are/is determined from the difference or the quotient of the actual engine rotational speed of the traction vehicle and a predefined target engine rotational speed or the pressure ratios in a hydrostatic traction drive of the traction vehicle. In addition, the maximum braking duration of the trailer brakes during automatic braking triggered by pushing of the trailer onto the traction vehicle is limited and the braking effect of the trailer brakes is adapted as a function of the velocity.
If the pressure ratios in a hydrostatic traction drive of the traction vehicle are used as a parameter for detecting the pushing of the trailer onto the traction vehicle, this procedure can be applied only in traction vehicles which also have such a hydrostatic traction drive. Use of the engine rotational speed as a parameter has the disadvantage in agricultural vehicles with the infinitely variable transmission described above that the driver can bring about deceleration solely by activating the operator control lever (joystick) of the infinitely variable transmission in order to change the transmission ratio without, however, changing the position of the accelerator pedal and therefore the engine rotational speed. In such a case, the trailer would then be unbraked.